This specification relates to devices and techniques of coupling radiation energy to a light sensing array, and more particularly, to radiation coupling to a quantum-well infrared sensing array via evanescent waves.
Quantum-well semiconductor devices can be designed to respond to radiation energy to produce charge indicative of the amount of received radiation. The radiation-induced charge can then be converted into an electrical signal (e.g., a voltage or current) to be processed by signal processing circuitry. Many quantum-well devices use an intraband transition between a ground state and an excited state in the same band (i.e., a conduction band or a valance band) of the quantum-well structure to detect infrared ("IR") radiation. The compositions of lattice-matched semiconductor materials of the quantum well layers can be adjusted to cover a wide range of wavelengths for infrared detection and sensing. In comparison with other radiation detectors, quantum-well structures can achieve a high quantum efficiency, a low dark current, compact size and other advantages. Infrared quantum-well sensing arrays may be used for various applications, including night vision, navigation, flight control, environmental monitoring.
A quantum well infrared sensor only responds to incident radiation with a polarization that is perpendicular to the plane of the quantum well layers. This is because only this polarization can induce an intraband transition at a desired infrared wavelength. Hence, the direction of the electric field of the received radiation must be parallel to the growth direction of the quantum well layers. One direct approach for light coupling is to orient the quantum well infrared photodetector at an angle to the incident infrared radiation (e.g., forty-five degree). The incident electric field will have a component along the growth direction of the quantum well layers to produce absorption of photons. Any additional scattering can enhance this absorption.
For applications based on imaging at focal plane arrays, the photodetector array is often oriented perpendicular to the scene to be imaged. Since the electric vector is essentially parallel to the quantum well layers in this arrangement, the quantum well layers absorb little or no light. One way to provide proper coupling is to use a random surface to scatter the incident radiation into the correct polarization for absorption. Alternatively, grating couplers with one or two-dimensional periodic profiles can be used to convert normally-incident radiation to waves propagating parallel to the quantum well layers.